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I have a M38 motor that I'm putting back together - it is a late version with the hole in the flywheel for a timing mark and an embossed rib on the timing gear cover to indicate TDC. Problem is, there is no 5 degree BTDC mark on the timing gear cover. The head is off and I was wondering if I can determine somehow where the mark should go using piston position or pulley position and scribe a mark myself. Any ideas?

That's the best time to do it. I assume you mean front crank pulley has a hole in it for timing since you said you have a late engine.

Rotate the crank in it's normal rotation until #1 piston is headed downward and the #1 intake valve is open. Stop just as you reach bottom and start upward. Now hook up a dial indicator on top of the block with it's plunger reaching into the bore about an 1/8" and slowly continue bringing the piston up until it stops moving the dial indicator needle upwards. That is TDC. Now mark TDC on your cover or a tab. Then measure 5 degrees of circumference of the pulley backwards and mark your 5 deg BTDC timing mark.

Yes, I meant the hole is in the pulley. The flywheel I have does not have any timing marks on it either but that is correct for a late M38 engine flywheel. The flywheel is not off of this engine anyway - I've got a mix of parts from several different engines. I'm guessing the timing gear cover is not original to the block as it doesn't have any marks other than the embossed rib.